override net80211's ic_node_alloc function to allocate a full

override net80211's ic_node_alloc function to allocate a full
bwi_node structure (containing the rate control state).
because bwi(4) does not support HostAP or IBSS modes there is
no need to maintain a per-node rate control state, so we could
as well store it in bwi_softc but that will allow for future
improvements.

pointed out by Taylor R Campbell (campbell AT mumble DOT net)
on tech@